Apparatus for pasteurizing liquid whole egg products

ABSTRACT

Disclosed is a method of pasteurizing a liquid whole egg product in a continuous stream. The method comprises the steps of: (a) providing an egg yolk product stream and an egg white product stream; (b) heating the egg yolk product stream to a predetermined temperature greater than the highest temperature of the egg white product stream; and then (c) recombining the egg yolk product stream and the egg white product stream to form a whole egg product stream, the whole egg product stream having (e.g., equilibrating to) a second predetermined temperature; wherein the total thermal treatment received by the liquid whole egg product during the process is at least sufficient to pasteurize the product. An apparatus useful for carrying out the method of the present invention is also disclosed.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a divisional application of co-pending priorapplication Ser. No. 07/468,066, filed with the U.S. Patent Office onJan. 23, 1990, now U.S. Pat. No. 5,019,407.

BACKGROUND OF THE INVENTION

The present invention concerns a method and apparatus for pasteurizingliquid whole egg products in which the product is split into two streamsfor initial heat treatment and then recombined for completion of heattreatment.

Large quantities of liquid egg products are pasteurized each year toproduce products sold in refrigerated form with limited shelf life. Seegenerally, Egg Pasteurization Manual (USDA Agricultural Research Service1969). More recently, the provision of techniques for ultrapasteurizingliquid egg products has led to considerable interest in marketing liquidegg products which have extended shelf lives under refrigeratedconditions.

M. Hamid-Samimi et al., in IUFOST Symposium on Aseptic Processing &Packaging of Foods Proceedings, 229 (Symposium held Sep. 9-12, 1985, inTylosand, Sweden), concerns ultrapasteurizing liquid egg in which theegg is heated by contacting to a heated surface. See also U.S. Pat. No.4,808,425, issued Feb. 28, 1989.

U.S. Pat. No. 3,113,872 to Eynon Jones discloses a method of treatingliquid egg products (shown schematically in FIG. 2 therein) whichemploys (a) a first heating step in which liquid egg is contacted to aheated surface, (b) a first vacuum evaporation and cooling step, (c) asecond heating step in which liquid egg is contacted to steam, and (d) asecond vacuum evaporation and cooling step.

T. Sugihara et al., Food Technology 20. 1 (1966) discloses a method oftreating liquid whole egg (shown schematically in FIG. 1 on page 4thereof) which employs (a) a first heating step in which liquid egg iscontacted to a heated surface, (b) a second heating step in which theliquid egg is contacted to steam, and (c) a vacuum evaporation andcooling step. L. Kline et al., Food Technology, 105 (November 1965), atpage 114, report a method of processing liquid egg white similar to themethod of processing liquid whole egg disclosed by Sugihara et al.,supra.

Co-pending patent application Ser. No. 312,066 of Swartzel, Ball, andLiebrecht, titled "Ultrapasteurization of Liquid Whole Egg Products withDirect Heat," and filed Feb. 16, 1989, discloses a method ofultrapasteurizing a liquid whole egg product in which the product isinitially heated to a first predetermined temperature in a first heatingstep. The product is then maintained at the first predeterminedtemperature for a first predetermined holding time. Next, the product isheated to a second predetermined temperature in a direct heating step.The product is then maintained at the second predetermined temperaturefor a second predetermined holding time sufficient to cause the desiredbacterial kill in the product. The product is then cooled andaseptically packaged to provide a packaged liquid whole egg producthaving a shelf life of at least four weeks under refrigeratedconditions. In a preferred embodiment of the invention disclosedtherein, the product is heated by steam injection in the direct heatingstep and then cooled without the use of a vacuum chamber.

Ultrapasteurization processes which contact the liquid whole egg productto a heated surface produce products of excellent quality, but havelimited run times due to eventual fouling of the heated surface.Ultrapasteurization processes which contact the liquid whole egg productto steam make longer run times available, but the relative severity ofthe steam contact step usually results in a lower quality product. Inview of the significant demand for ultrapasteurized liquid whole eggproducts, there is a need for new ultrapasteurization processes whichprovide greater control over the process so as to make longer run timeswith higher quality products possible. The present invention is based onour ongoing efforts to provide improved techniques for pasteurizing andultrapasteurizing such products.

SUMMARY OF THE INVENTION

Disclosed is a method of pasteurizing a liquid whole egg product in acontinuous flow apparatus. The method comprises the steps of: (a)providing an egg yolk product stream and an egg white product stream;(b) heating the egg yolk product stream to a predetermined temperaturegreater than the highest temperature of the egg white product stream;and then (c) recombining the egg yolk product stream and the egg whiteproduct stream to form a whole egg product stream, the whole egg productstream having (e.g., equilibrating to) a second predeterminedtemperature; wherein the total thermal treatment received by the liquidwhole egg product during the process is at least sufficient topasteurize the product. Preferably, the whole egg product stream is heldat the second predetermined temperature for a time sufficient to cause anine log cycle reduction in Listeria monocytogenes in the liquid wholeegg product.

The step of recombining the egg yolk product stream and the egg whiteproduct stream is preferably carried out by injecting either the eggyolk product stream into the egg white product stream, or the egg whiteproduct stream into the egg yolk product stream.

The method may optionally include the step of removing cholesterol fromthe egg yolk product stream before the step of recombining the egg yolkproduct stream and the egg white product stream.

An apparatus for pasteurizing a liquid whole egg product is alsodisclosed. The apparatus comprises (a) an egg yolk product line; (b) anegg white product line; (c) first pumping means connected to the eggyolk product line for establishing an egg yolk product stream in the eggyolk product line; (d) second pumping means connected to the egg whiteproduct line for establishing an egg white product stream in the eggwhite product line; (e) a heater connected to the egg yolk product linefor heating the egg yolk product stream to a first predeterminedtemperature; (f) recombining means connected to the egg yolk productline after the heater and connected to the egg white product line forproducing a whole egg product stream from the egg yolk product streamand the egg white product stream; and (g) a whole egg product lineconnected to the recombining means for receiving the whole egg productstream.

The method and apparatus of the present invention enable eachconstituent of the liquid whole egg product to be heated in a mannermost appropriate for that constituent. With the present apparatus,holding times and temperatures can be tailored to each constituentstream. By providing the final heat-imparting step to the egg whiteproduct stream by means of recombining it with an egg yolk productstream heated to a higher temperature, the more heat-sensitive egg whiteconstituent of the final product can be heated to the highesttemperature it attains during the process without contacting it toeither a heated surface or steam.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE schematically illustrates an apparatus for carrying out thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Examples of pasteurized liquid whole egg products which may be producedby the method of the present invention include whole egg, fortifiedwhole egg (whole egg with added yolk), salt whole egg (e.g., salt 10%),sugar whole egg (e.g., sugar 10%), blends of whole egg with syrupsolids, syrups, dextrose and dextrins and/or gums and thickening agents,blends of whole eggs with less than 1% sugar and/or salt, scrambled eggmixes (for example, a mix of about 51% egg solids, 30% skim milk solids,15% vegetable oil and 1.5% salt), reduced cholesterol egg products andblends thereof, custard blends, and the like, that is, productscontaining at least about 10% egg solids. Products which are extremelysensitive to thermal processing and which are particularly suitable forultrapasteurization by the present invention include, for example,liquid whole egg and blends thereof (less than 2% added non-eggingredients), fortified whole egg and blends thereof (24-38% egg solids,2-12% added non-egg ingredients), liquid salt whole egg, liquid sugarwhole egg, and other liquid whole egg blends which are 24-38% egg solidsand 12% or less of added non-egg ingredients. Terms used herein havetheir standard meaning in accordance with industry and regulatory usage.See. e.g., 7 C.F.R. §59.570(b) (1985).

Any means for establishing respective constituent streams (i.e., the eggyolk product stream and the egg white product stream) may be used tocarry out the present invention. Pumps used to establish streams may ormay not be positive displacement pumps, though positive displacementpumps (timing pumps) are generally needed to precisely define theholding time of a product stream in a holding section Positivedisplacement pumps may be used in combination with other pumping means,such as centrifugal pumps.

Ingredients may be added to or removed from the various product streamsin the present method at any location to provide the various liquidwhole egg products given above. The addition and removal of water, andthe removal of cholesterol, are discussed specifically below. In anotherembodiment of the invention, the liquid yolk product stream consistsonly of liquid egg yolk (cholesterol optionally removed), the liquidwhite product stream consists only of liquid egg white, and the liquidwhole egg product stream consists only of liquid whole egg (cholesteroloptionally removed). Minor ingredients, such as salt, or citric acid orphosphate to stabilize color, may be included in all embodiments.

At least the egg yolk product stream is heated prior to recombining itwith the egg white product stream. The egg white product stream mayoptionally be heated (and is preferably heated) prior to therecombination step, though it is not heated to as high a temperature asthe egg yolk product stream.

How the constituent product streams are heated is not critical Therespective product streams may be heated by contacting them to a heatedsurface (indirect heat), as described in U.S. Pat. No. 4,808,425 (thedisclosure of which is incorporated herein by reference), or heated bycontact to or by the injection of steam (direct heat), as described incommonly owned U.S. patent application Ser. No. 07/312,066, filed Feb.16, 1989 (the disclosure of which is incorporated herein by reference).Preferably, however, the egg white product stream, if heated, is heatedby contact to a heated surface rather than by contact to steam.

A cholesterol removal step may optionally be included in the process ofthe present invention. Since egg cholesterol is located in yolk, thecholesterol removal step need only be carried out on the yolk productstream. The particular cholesterol removal procedure employed is notcritical. One exemplary process employing cyclodextrins to bindcholesterol is disclosed in A. Bayol et al., European Patent ApplicationNo. 89400175.9, Process for Elimination of Steroids from a Substance ofBiological Origin (Laid open Aug. 2, 1989). Other exemplary processesare disclosed in U.S. Pat. Nos. 4,234,619, 3,882,034, and 3,607,304, thedisclosures of which are incorporated herein by reference. Preferably,however, cholesterol is removed by passing the yolk product streamthrough a chromatography column, which chromatography column containscyclodextrin immobilized on a solid support. Preferably β-cyclodextrinis immobilized on the solid support, and preferably the chromatographycolumn is positioned in the pasteurizing apparatus so that the yolkproduct stream is passed through the column before it is heated. Anysolid support suitable for use in processing food may be employed. Thesolid support may be a silica support, with the cyclodextrin immobilizedthereon in the manner described in U.S. Pat. No. 4,781,858 to F.Mizukami et al.; the solid support may be glass beads, with thecyclodextrin immobilized thereon in the manner described in U.S. Pat.No. 4,087,328 to Swaisgood (all patent references cited herein are to beincorporated herein by reference). The immobilized cyclodextrin mayoptionally be separated from the yolk product stream by a semipermeablemembrane permeable to the cholesterol, as described in U.S. Pat. Nos.4,714,556 and 4,361,484 (e.g., by passing the yolk product streamthrough the lumen of a capillary bed formed from a semipermeablemembrane material, with the cyclodextrin immobilized on a solid supportlocated on the opposite side of the capillary bed membrane).

There may optionally be holding sections included for each constituentproduct stream prior to the recombination step. It is not essential thatthe holding times imparted by these two sections to their respectiveconstituent streams be the same. In one embodiment of the invention, asdiscussed below, holding sections for constituent product streams arekept to a minimum prior to the recombination step. In another embodimentof the invention, also discussed below, holding sections prior to therecombination step are sufficient to impart a nine log cycle reductionin Salmonella to each constituent product stream, with the heattreatment after the recombination step sufficient to bring the totalthermal treatment imparted by the process to a level sufficient toproduce a shelf stable refrigerated product as described below (andpreferably sufficient to impart a nine log cycle reduction in Listeriato the product).

Constituent product streams may be recombined by any means, such as byemptying the two streams into a holding tank or mixing tank for apredetermined time. However, recombination of the constituent productstreams is preferably carried out by injecting one of the constituentproduct streams into the other. It is most convenient to inject thestream of smaller volume into the larger volume stream, which generallymeans injecting the egg yolk product stream into the egg white productstream. This transfers heat from the egg yolk product stream, which hasbeen raised to a temperature higher than the egg white product stream,to the more heat-sensitive egg white product stream. The whole eggproduct stream formed by this recombination is optionally temperatureequilibrated by an in-line static mixer in the liquid whole egg productline immediately after the recombining means, and before any holdingsection.

If desired, the transfer of heat from the egg yolk to the egg white canbe enhanced by adding water to the egg yolk product stream prior to itsfirst heating step, and then removing the water from the recombinedwhole egg product stream after the holding step. Water may be removed bymeans such as a vacuum chamber (which would also serve to cool theproduct) or reverse osmosis.

The total thermal treatment received by the liquid whole egg productduring the process (that is, both before and after the recombinationstep) must be sufficient to cause at least a nine log cycle reduction ofSalmonella bacteria in the product (i.e., sufficient to pasteurize theproduct). Proper thermal treatment is typically insured by presettingthe holding times, either of the constituent product streams prior totheir recombination, of the liquid whole egg product stream afterrecombination, or both. The term "holding time," as used herein, has itsordinary meaning as used in the industry, and all log cycle reductionsreferred to herein concern processed product as compared to raw orunprocessed product. Preferably, the thermal treatment is sufficient toproduce a product having a shelf life of about four weeks to aboutthirty-six weeks under refrigerated conditions, and more preferably aproduct having a shelf life of about eight weeks to about thirty-sixweeks under refrigerated conditions. The term "refrigerated," as usedherein, means stored at a temperature of 4° Centigrade.

Most preferably the holding time for the liquid whole egg product streamis sufficient to cause a nine log cycle reduction in Listeriamonocytogenes (Listeria) in the product. Part of this reduction may beprovided by thermal treatment carried out prior to the recombinationstep. In one embodiment of the invention, the thermal treatment carriedout on the constituent product streams prior to the recombination stepis sufficient to pasteurize the liquid whole egg product product, andthe thermal treatment carried out after the recombination step issufficient to insure that the total thermal treatment (before and afterrecombination) can cause a nine log cycle reduction in Listeria therein.Listeria is found in a wide range of animals (including man), on plants,and in soil. See Microbiology, 799-800 (B. Davis et al. Eds., 3d Ed.1980)(Harper & Row). A treatment capable of causing a nine log cyclereduction in Listeria is preferred because the wide-spread nature ofListeria makes it difficult to exclude from a pasteurizing plant,because of the pathogenic nature of this microorganism, see Id., andbecause Listeria appears capable of growing to large quantities in eggunder refrigerated conditions (the conditions contemplated for theproducts of the present process before they are prepared and eaten by aconsumer).

To obtain a product with reduced quantities of microorganisms, thepasteurizing apparatus should be sterilized before the liquid whole eggproduct is passed therethrough. Sterilizing is preferably accomplishedby passing hot water under pressure through the pasteurizing apparatus,as is known in the art, so that hot water is contacted to those surfaceswhich contact the product at a temperature and pressure and for a timesufficient to sterilize these surfaces.

In addition, the product, after pasteurization, should be asepticallypackaged. Aseptically packaged means packaged to the exclusion ofmicroorganisms other than those carried by the liquid egg productitself. Equipment suitable for aseptically packaging liquid egg productsis commercially available. Also useful in carrying out this step isequipment which packages the product to the substantial exclusion ofmicroorganisms, known in the industry as "clean fillers," but thegreater exclusion of microorganisms provided by aseptic fillers makesaseptic fillers preferable. This is particularly the case in view of theability of Listeria to grow under refrigerated conditions, as discussedabove.

A homogenization step may optionally be included after the product hasbeen maintained at the second predetermined temperature. If a vacuumevaporation and cooling step is included, it is preferably positionedbetween the recombination step and the homogenization step. The term"homogenize," as used herein, means to subject a product to physicalforces to reduce particle size. Such procedures are known in the art,and may be carried out on different types of equipment. It is preferableto carry out this homogenizing step with homogenizing equipment at totalpressures of from about 500 p.s.i. to about 3,000 p.s.i.

An apparatus for carrying out the method of the present invention isshown schematically in the FIGURE. This apparatus is readily assembledfrom commercially available parts. The apparatus of the FIGURE comprisesa 150 gallon raw product holding tank 1, 1' for each constituent productline, one line serving as the egg yolk product line for carrying the eggyolk product stream and the other line serving as the egg yolk productline for carrying the egg white product stream. All product linesinterconnecting the various portions of the apparatus are formed ofconventional sanitary stainless steel tubing. A centrifugal 2, 2' pumpfor each each holding tank is provided to pump each constituent out ofits respective holding tank. A Cherry-Burrell Model E Superplate heater3, 3' for each constituent product line is provided to bring eachconstituent product line to a predetermined temperature. A positivepiston pump 4, 4' is positioned in each product line after the heater,and a check valve 5, 5' is positioned in each product line after thepositive displacement pump and before injector 10. The lengths of theconstituent product lines between the heaters and injector are minimized(though they need not be, and need not be equal).

A Cherry-Burrell UHT I-type steam injection heater serves as injector 10and is connected to each constituent product line so that the egg yolkproduct stream carried in one product line is injected into the eggwhite product stream carried in the other product line Thus, the eggyolk serves as the heating media rather than steam, as is conventionalwith a steam injection heater.

A liquid whole egg product line emenates from the injector. A holdingsection 11 in the liquid whole egg product line follows the injector,and a flow diversion valve 12 in the liquid whole egg product linefollowing the holding section is provided for shunting improperlytreated product to a holding tank (not shown) via a diversion line 13.The length of the holding section 11 is chosen to provide the desiredholding time to the liquid whole egg product stream.

A Cherry-Burrell Model HD-6 aseptic homogenizer 14 in the liquid wholeegg product line follows the flow diversion valve, a Cherry-BurrellModel 558X8 No-Bac Spiratherm tube-in-shell cooler 15 in the whole eggproduct line follows the aseptic homogenizer, and a Tetra Pak ModelAB-3-250 aseptic filler 20 receives the whole egg product line andaseptically packages the whole egg product stream carried therein inindividual cartons.

Liquid egg yolk and liquid egg white may be provided as constituentingredients by conventional shell egg breaking apparatus, such as aSeymore breaker (not shown). The breaking apparatus may be in the sameplant as the pasteurizing apparatus of the invention or in a separateplant.

In use for carrying out a method of the present invention, the apparatusof the FIGURE may, for example, be set so that the heater 3 in the eggwhite product line heats the egg white product stream to about 62°Centigrade and the heater 3' in the egg yolk product line heats the eggyolk product stream to about 78° C. The positive displacement pumps 4,4' for the different product lines are set to pump at differentpredetermined rates so that the constituent product streams arerecombined to form a liquid whole egg product containing egg yolk andegg white in the same ratio as in natural whole egg. The constituentproduct streams recombine to provide a liquid whole egg product streamof about 67° C. The holding section 11 is a length suitable to provide aholding time for the liquid whole egg product stream of about 2.5minutes to thereby produce a pasteurized liquid whole egg product. Toproduce a product with greater kills of Listeria, the holding section islengthened to give a holding time of 3.5 minutes.

A second apparatus for carrying out the method of the present inventionis essentially the same as that shown in the FIGURE, except that achromatography column containing β-cyclodextrin immobilized on a silicasupport (as discussed in detail above) is positioned in the egg yolkproduct line after the pump 2' and before the heater 3'. This apparatusis thereby adapted to produce a reduced cholesterol product.

Those skilled in the art will appreciate that minor variations can bemade in the apparatus and procedures described herein without departingfrom the spirit of the present invention. Thus, the invention is definedby the following claims, with equivalents of the claims to be includedtherein.

We claim:
 1. An apparatus for pasteurizing a liquid whole egg product,comprising:(a) an egg yolk product line; (b) an egg white product line;(c) first pumping means connected to said egg yolk product line forestablishing an egg yolk product stream in said egg yolk product line;(d) second pumping means connected to said egg white product line forestablishing an egg white product stream in said egg white product line;(e) a heater connected to said egg yolk product line for heating saidegg yolk product stream to a first predetermined temperature; (f)recombining means connected to said egg yolk product line after saidheater and connected to said egg white product line for producing awhole egg product stream from said egg yolk product stream and said eggwhite product stream; and (g) a whole egg product line connected to saidrecombining means for receiving said whole egg product stream.
 2. Anapparatus according to claim 1, further comprising a holding section insaid whole egg product line.
 3. An apparatus according to claim 2,further comprising an aseptic filler connected to said whole egg productline.
 4. An apparatus according to claim 1, wherein said recombiningmeans comprises an injector.
 5. An apparatus according to claim 1,wherein said heater comprises a heated surface heat exchanger.
 6. Anapparatus according to claim 1, further comprising a chromatographycolumn connected to said egg yolk product line, said chromatographycolumn containing cyclodextrin immobilized on a solid support.
 7. Anapparatus according to claim 1, wherein each of said first and secondpumping means comprises a positive displacement pump.